BackgroundOne of the hallmarks of multicellular organisms is the ability of their cells to trigger responses to the environment in a coordinated manner. In recent years primary cilia have been shown to be present as ‘antennae’ on almost all animal cells, and are involved in cell-to-cell signaling in development and tissue homeostasis; how this sophisticated sensory system arose has been little-studied and its evolution is key to understanding how sensation arose in the Animal Kingdom. Sponges (Porifera), one of the earliest evolving phyla, lack conventional muscles and nerves and yet sense and respond to changes in their fluid environment. Here we demonstrate the presence of non-motile cilia in sponges and studied their role as flow sensors.ResultsDemosponges excrete wastes from their body with a stereotypic series of whole-body contractions using a structure called the osculum to regulate the water-flow through the body. In this study we show that short cilia line the inner epithelium of the sponge osculum. Ultrastructure of the cilia shows an absence of a central pair of microtubules and high speed imaging shows they are non-motile, suggesting they are not involved in generating flow. In other animals non-motile, ‘primary’, cilia are involved in sensation. Here we show that molecules known to block cationic ion channels in primary cilia and which inhibit sensory function in other organisms reduce or eliminate sponge contractions. Removal of the cilia using chloral hydrate, or removal of the whole osculum, also stops the contractions; in all instances the effect is reversible, suggesting that the cilia are involved in sensation. An analysis of sponge transcriptomes shows the presence of several transient receptor potential (TRP) channels including PKD channels known to be involved in sensing changes in flow in other animals. Together these data suggest that cilia in sponge oscula are involved in flow sensation and coordination of simple behaviour.ConclusionsThis is the first evidence of arrays of non-motile cilia in sponge oscula. Our findings provide support for the hypothesis that the cilia are sensory, and if true, the osculum may be considered a sensory organ that is used to coordinate whole animal responses in sponges. Arrays of primary cilia like these could represent the first step in the evolution of sensory and coordination systems in metazoans.

There was an error published in J. Exp. Biol. 220, 995-1007. Some values for head loss and respiration in Table 3 were carried over from an earlier version of the manuscript. The corrected table follows.The final numbers for volume flow rate, head loss, pumping power and cost of pumping remain unchanged, and there are no changes to the results and conclusions of the paper. The data available from the University of Alberta Education Resource Archive (ERA; https://doi.org/ 10.7939/R36688W8N) are correct. The authors apologise for any inconvenience this may have caused. 4743© 2017. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2017) 220, 4743-4744 doi:10.1242 Journal of Experimental Biology ΔH and measured volume flow rate are used to calculate the pumping power P p using Eqn A6. The cost of pumping η (%) is then estimated using Eqn A7 from the pumping power P p and the measured respiration rate R tot . The collar slit is in bold, representing the filtration apparatus. Journal of Experimental Biology (2017) 220, 4743-4744 doi:10.1242 Journal of Experimental Biology ABSTRACT Sponges (Porifera) are abundant in most marine and freshwater ecosystems, and as suspension feeders they play a crucial role in filtering the water column. Their active pumping enables them to filter up to 900 times their body volume of water per hour, recycling nutrients and coupling a pelagic food supply with benthic communities. Despite the ecological importance of sponge filter feeding, little is known about how sponges control the water flow through their canal system or how much energy it costs to filter the water. Sponges have long been considered textbook examples of animals that use current-induced flow. We provide evidence that suggests that some species of demosponge do not use currentinduced flow; rather, they respond behaviourally to increased ambient currents by reducing the volume of water filtered. Using a morphometric model of the canal system, we also show that filter feeding may be more energetically costly than previously thought. Measurements of volumetric flow rates and oxygen removal in five species of demosponge show that pumping rates are variable within and between species, with the more oxygen consumed the greater the volume filtered. Together, these data suggest that sponges have active control over the volume of water they process, which may be an adaptation to reduce the energetic cost of filtration in times of high stress. 4744 CORRECTION

There was an error published in J. Exp. Biol. 220, 995-1007. Some values for head loss and respiration in Table 3 were carried over from an earlier version of the manuscript. The corrected table follows.The final numbers for volume flow rate, head loss, pumping power and cost of pumping remain unchanged, and there are no changes to the results and conclusions of the paper. The data available from the University of Alberta Education Resource Archive (ERA; https://doi.org/ 10.7939/R36688W8N) are correct. The authors apologise for any inconvenience this may have caused. 4743

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